This invention relates to an oxygen depletion and removal system (ORS) and a nitrogen generating system (NGS). This invention also relates to an on board inert gas generation system (OBIGGS), flammability reduction system (FRS), nitrogen generating system (NGS), and fuel tank inerting system (FTIS).
There are risks associated with the flammability of a fuel tank ullage composition. One approach to reducing the risk is to supply inert gas into the fuel tank. This reduces the oxygen concentration in the ullage and achieves an oxygen/fuel vapor ratio below the lower explosion limit (LEL).
Two ways to supply inert gas are by: 1) generating a nitrogen-enriched air (NEA) stream from outside air using an air separation membrane; and 2) generating a carbon dioxide stream from fuel. The first approach of using a membrane requires heavy, expensive membrane modules that require high pressure, cool, filtered air to operate efficiently. High pressure air is obtained from dedicated compressors, which add weight and use power; or from engine bleed air, which incurs a fuel penalty for the airplane. Cooling the air may require heat exchangers. Filtering the air requires removal of ozone and liquid and solid particulates by filters that require replacement.
The second approach, using CO2 has certain disadvantages. The production of CO2 consumes fuel. The fuel may also contain sulfur, which may form corrosive sulfur oxides. The production of CO2 from fuel also produces water, which needs to be removed from the carbon dioxide stream before being returned to the fuel tank. The second approach may also require a heater to operate efficiently.
The prior art includes a process discussed at a presentation in New Jersey on 2 Nov. 2005, titled Next Generation OBIGGS: Developments at Phyre Technologies. This process reduces oxygen content in the fuel tank by using oxygen as an oxidizing agent to produce CO2 and water. The process involves high temperature for catalytic oxidation and, as a result, a heat exchanger is needed to reduce the heating requirement and to cool the product before entering the fuel tank.
U.S. Pat. No. 3,847,298 (“the '298 patent”) discloses the inerting of aircraft ullage by catalytic oxidation of liquid fuel with bleed air. FIG. 1 of the '298 patent illustrates the general process. A reactor is used to produce gas such as CO2 and H2O. The gas is then cooled by a cooling system or gas cooler. After the gas is cooled, it enters a moisture remover to remove H2O before the gas is recycled back to the fuel tank. The catalytic reaction occurs at high temperature −“500°/600° F.”—as described at column 3, line 10 of the '298 patent. Moreover, the reactor operates in an open loop using engine bleed air to react with fuel from the tank.
U.S Patent Publication No. 2002/0028168 discloses inert gas production by combustion of an organic hydrocarbon fuel. This production is in an internal combustion engine followed by a catalytic process to substantially remove oxygen. As shown in FIG. 1, an exhaust stream from the engine passes through a heat exchanger to cool the exhaust gas, and the cooled exhaust gas stream is compressed with a compressor. The compressed gas is then heated and additional fuel is introduced into the exhaust gas stream. The heated and pressurized exhaust gas stream is then passed through a packed bed catalytic system to convert oxygen in the exhaust gas to CO2 and water vapor and thus produce inert gas containing less than 1000 ppm of oxygen.
As can be seen, there is a need for a low temperature, closed loop flow circuit that draws the ullage from the tank and passes the flow through the catalyst. There is also a need for use of a lighter, smaller catalyst; and a more cost-efficient way to reduce the oxygen/fuel vapor ratio in a fuel tank ullage that does not generate undesired by-products or use high pressures or temperatures, and does not create a fuel penalty by consuming fuel or using engine bleed air.